Background: Increased head circumference and body size, as well as the presence of high levels of growth-related hormones, have been reported in autism spectrum disorders (ASD). Anthropomorphic differences may provide a clue to the underlying biology of autism.

Objectives: To compare birth weight of children with ASD, their unaffected siblings, and the general U.S. population.

Methods: Self-report data on birth weight of 4,600 children with ASD and 3,978 of their unaffected siblings was collected from families using a web-based interface. U.S. population vital statistics for 2005 were used for purposes of comparison. Birth weight was divided into five categories: Very Low (<1500 grams), Moderately Low (1500-2499 grams), Normal (2500-3999 grams), Moderately High (4000-4500 grams), and Very High (>4500 grams). Chi square analyses were performed.

Results: Compared with children of the same gender in the general U.S. population, boys and girls with ASD, as well as their male and female siblings, were more likely to have a Moderately High or Very High birth weight. 16.1% of boys with ASD and 17.6% of male siblings had a high birth weight, while only 10.5% of all male children (and 12.5% of white, non-Hispanic boys) did so. 10.9% of girls with ASD and 10.5% of female siblings likewise had a high birth weight, while only 6.3% of all female children (and 7.3% of white, non-Hispanic girls) did so. (For all comparisons, p<.001.)

Conclusions: Preliminary data suggest that both children with ASD and their siblings are more likely to have a Moderately High or Very High birth weight when compared with children of the same gender in the general U.S. population. These findings should be further explored using other data sources and controlling for additional potential confounders.